1. Field of the Invention
The present invention relates to an ink jet head and an ink jet printer performing an image formation by ejecting an ink droplet.
2. Discussion of the Background
In a conventional technique, a shear mode ink jet head has been well-known as disclosed in U.S. Pat. No. 4,879,568 wherein a capacity in a pressure chamber is varied by pressure means that produces a shear strain in accordance with an electrical signal for selectively ejecting ink from an ejecting nozzle provided at each pressure chamber, thereby performing an image formation. This type of shear mode ink jet head has a characteristic that the pressure chamber is easy to be arranged with high density.
However, the above-mentioned shear mode ink jet head has a problem that a phenomena so-called crosstalk occurs in which a pressure fluctuation in some pressure chamber gives a fluctuation in a pressure or a flow velocity of the ink in the other nearby pressure chamber. It is considered that the crosstalk occurs because the pressure of the ink in the pressure chamber displaces a partitioning wall between the pressure chambers to thereby change the ink pressure in the adjacent and nearby pressure chambers.
Pressure chambers at the side of both ends within a printing range receive the crosstalk from only the other pressure chambers positioned at the inside within the printing range, while the pressure chambers positioned at the inside of the printing range receive the crosstalk from the other pressure chambers positioned at both sides. Therefore, the influence by the crosstalk is different between the pressure chambers positioned at both sides within the printing range and the pressure chambers positioned at the inside thereof. This leads to a difference between a volume of the ink droplet ejected from an ejecting nozzle communicating with the pressure chambers positioned at both sides within the printing range and a volume of an ink droplet ejected from an ejecting nozzle communicating with the pressure chambers positioned at the inside of the printing range, thereby being likely to cause a non-uniform density or a deterioration in image quality in a printed matter.
There is an ink jet head of FIG. 13 disclosed in, for example, Japanese Unexamined Patent Application No. 2000-135787 as an ink jet head aiming to establish an equalization of the influence of the crosstalk exerted on each pressure chamber. The ink jet head shown in FIG. 13 has three dummy pressure chambers 102 formed respectively at both sides of plural pressure chambers 101 arranged in a printing range, each pressure chamber 101 having a single ejecting nozzle 103 communicating therewith and each dummy pressure chamber 102 having plural dummy nozzles 104 communicating therewith. The “dummy pressure chamber” means herein a pressure chamber from which ink is not ejected even if a driving signal is applied.
When for example, an ink droplet is ejected by changing the capacity in the pressure chamber 101a positioned at the edge section within the printing range in the ink jet head shown in FIG. 13, the dummy pressure chamber 102a similarly changes its capacity simultaneous with the ejection of the ink droplet. Further, when an ink droplet is ejected by changing the capacity in the pressure chamber 101b positioned at the edge section within the printing range, the dummy pressure chamber 102b similarly changes its capacity simultaneous with the election of the ink droplet. Further, when an ink droplet is ejected by changing the capacity in the pressure chamber 101c positioned at the edge section within the printing range, the dummy pressure chamber 102c similarly changes its capacity simultaneous with the ejection of the ink droplet.
This enables to exert the influence of the crosstalk from the other pressure chambers (effective pressure chamber and dummy pressure chamber) positioned at both sides on the pressure chambers 101a, 101b and 101c positioned at the edge sections within the printing range, like the other pressure chambers positioned at the inside of these pressure chambers 101a, 101b and 101c. 
However, the ink jet head shown in FIG. 13 has plural dummy nozzles 104 communicating with one dummy pressure chamber 102 in order not to eject an ink droplet from the dummy nozzles 104 in case where the capacity in the dummy pressure chamber 102 is charged.
Therefore, a flow impedance of the dummy nozzle 104 for the dummy pressure chamber 102, i.e., a viscosity resistance, inertial resistance or the like of the ink produced at the dummy nozzle 104 reduces in inverse proportion to the number of the dummy nozzle 104. As a result, a main acoustic resonance frequency of the ink in the dummy pressure chamber 102 differs from that of the ink in the pressure chamber 101.
The main acoustic resonance frequency is a frequency in which, when the pressure chamber is driven by applying voltage with the pressure means, a pressure wave occurring in the ink in the pressure chamber is transmitted through the ink in the pressure chamber and is overlapped to thereby become the greatest pressure vibration. This frequency is called a Helmholtz resonance frequency.
Therefore, when a driving signal having a waveform matched to the acoustic resonance frequency of the ink in the effective pressure chamber 101 is applied to the ink in the dummy pressure chamber 102, an extraordinary pressure fluctuation occurs in the dummy pressure chamber 102, whereby the crosstalk caused by the extraordinary pressure fluctuation occurring in the dummy pressure chamber 102 is exerted on the respective three effective pressure chambers 101 positioned at both end sections within the printing range, thereby rather entailing a problem of bringing non-uniform density or deterioration in image quality depending upon the situation.